The innate immune system is the first line of defense against pathogens. Innate immune cells lack the exquisite specificity of the adaptive immune system, yet in order to respond in a measured way they must be able to tailor their response to the specific pathogen. These cells have therefore evolved pattern recognition receptors (PRRs) that recognize conserved molecular patterns characteristic of the microbe, which are not found within the host. The Toll-like receptors (TLRs) and the RIG-I-like receptors (RLRs) are the PRRs that detect viruses. These receptors initiate anti-viral responses principally by inducing type I interferons. The tight regulation of type I IFNs is critical since overproduction of the cytokine can contribute significantly to autoimmune disease;thus the balance between health and disease is determined by exquisite regulation of this pathway. The transcription factor IRF7 is a "master regulator" of systemic type 1 IFN responses. While much is known about the mechanisms by which IRF7 is activated little is known about its negative regulation. Systems biology approaches have enabled us to identify at least two negative regulators of the IRF7 pathway: the transcription factor Foxo3 that regulates IRF7 at a transcriptional level and microRNA-144 (miR-144) that regulates the IRF7 network post-transcriptionally. We also have evidence that at least one additional miRNA regulates IRF7. This proposal aims to clarify the molecular mechanisms by which Foxo3 and miR-144 control IRF7. It also aims to identify the unknown IRF7-regulating miRNAs using a high throughput screen. Systems approaches will then be used to integrate Foxo3 and the miRNAs into a global IRF7 regulatory network. Finally, we will examine the in vivo relevance of the IRF7- FOXO3-miRNA regulatory circuit in a mouse model of VSV infection. PUBLIC HEALTH RELEVANCE: The proteins that are being investigated have a pivotal role in regulating the immune response in people. They instruct the body to effectively combat infectious disease, and an understanding of their function will permit us to make better vaccines and drugs. However, these molecules are a two-edged sword. When they function incorrectly they lead to inflammatory and autoimmune diseases. Understanding how this happens will lead to the design and production of better drugs for diseases such as rheumatoid arthritis.